This invention relates to improvements in a low-friction sliding member in a transmission, and more particularly to the low-friction sliding member suitable to be used in a continuously variable transmission and in an automatic transmission and a transmission oil composition used for the low-friction sliding member.
Global environmental problems such as global warming and ozone layer destruction are coming to the fore. As it has been said that the global warming is significantly affected by CO2 emission, the reduction of CO2 emission, notably the setting of CO2 emission standards, has become a big concern to each country. One of challenges to reduce CO2 emission is to improve a fuel efficiency or fuel economy that depends on the performance of sliding members and a lubricating oil applied thereto. It will be understood that the sliding members and the lubricating oil perform great roles for improving the fuel economy.
The sliding members have been required to be excellent in wear resistance even in a sliding site where a frictional wear circumstance is severe as compared with other sliding sites in the transmission. Recently, application of a variety of hard thin films has been in progress. In general, hard carbon thin films have been expected as low-friction materials for the sliding members because they have low friction coefficients in air and in presence of lubricating oil as compared with wear-resistant hard coating materials such as titanium nitride (TiN) or chromium nitride (CrN).
Additionally, there are the following approaches to improve the fuel economy in connection with the lubricating oil: (1) lowering the viscosity of the lubricating oil, thereby reducing viscous resistance in hydrodynamic lubrication regions and agitation resistance in the engine; and (2) adding a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication.
Such approaches concerning frictional characteristics in connection with the lubricating oil are discussed in in Japan Tribology Congress 1999. 5, Tokyo, Proceeding Page 11-12, KANO et al and in World Tribology Congress 2001. 9, Vienna, Proceeding Page 342, KANO et al.